Availability of sources of coherent electromagnetic radiation at wavelengths in the range of 0.5-15 micrometers and of waveguides for transmitting such radiation has produced widespread interest in the use of light as a communications medium. Proposed optical communications systems may comprise light sources, waveguides, and active components such as, e.g., modulators, deflectors, and switches. Waveguides may be of a type known as optical fibers or else they may take the form of patterned transparent films on a substrate, the latter type being particularly suited for miniaturized, highly complex systems.
Such systems are described, e.g., by S. E. Miller, "Integrated Optics: An Introduction", Bell System Technical Journal, Vol. 48 (1969), pp. 2059-2069, and they may be manufactured by exemplary techniques as described by E. G. Spencer et al., "Ion-Beam Techniques for Device Fabrication", Journal of Vacuum Science and Technology, Vol. 8 (1972), pp. S52-S70.
Active components for integrated optical circuits have been proposed based on a variety of physical effects such as, e.g., electro-optical, magneto-optical, and acousto-optical interaction; among electro-optical devices are devices which are based on electrochromic and electroplating action as disclosed, respectively, in U.S. patent applications Ser. No. 973,162 and 973,188, filed Dec. 26, 1978 by L. F. Johnson et al.
It is recognized that photodetectors in optical communications systems are capable of operating over a wide range of power levels. This is in contrast to crystal detectors in microwave communications systems which may preferably be protected by power limiters comprising microwave attenuators as disclosed in U.S. Pat. No. 3,185,945, issued May 25, 1965 to W. H. Wright. There are instances, however, where it may be desirable to optically terminate a waveguide such that signals are removed from a network without the introduction of spurious signals as may be produced by reflection at a terminal.